This application relates to an improvement to vary the drag associated with an elevator car and counterweight, particularly as each approach extreme ends of their travel range.
Elevators are typically provided with an elevator car that moves upwardly and downwardly within an elevator shaft. As is known, and as shown somewhat schematically in FIG. 1, the car 12 is balanced by a counterweight 14. The two are connected by a rope 16. Sheaves 18 and 20 guide the rope.
In the prior art, and particularly as the elevator car or counterweight reach extreme ends of travel, one of the two becomes lighter than the other. The hoist rope is more on one side and thus the hoist rope adds more weight to the lower of the hoist car and counterweight. This raises some complications and challenges in providing smooth travel for the car 12. In particular, in very high rise applications, the provision of adequate counterweight balance forces at extreme positions of the counterweight or car becomes very challenging. Thus, compensating ropes 22 have sometimes been utilized. It would be desirable to eliminate the compensating ropes, such as rope 22.
Other issues with regard to the control of movement of an elevator car and counterweight include a problem known as “releveling” in which the car may move slightly once stopped at a floor. Further, a condition known as “counterweight jump” or “car jump” can occur at the end of travel if the car or counterweight strikes a buffer in the pit. The other of the car or counterweight may continue in an upward direction due to stored inertia. The rope tension on the lower element may become low such that its rope can become slack resulting in slip at the traction sheave. As the counterweight subsequently drops, the rope will become taut again.
Thus, the present invention eliminates the need for compensating ropes, and provides smoother travel for the elevator particularly at extreme ranges of travel.